Why "douche bag"?
Douche is like the italian word "doccia", the original meaning is" tube".

the explanations in the dictionary are
1 a : a jet or current of liquid (as a cleansing solution) directed against or into a bodily part or cavity (as the vagina) b : an act of cleansing with a douche
2 : a device for giving douches
3 British : an abrupt often chastening shock to the nerves, emotions, or awareness

so.. basically it's nothing related to an unattractive or offensive person...
Why "douche bag" ? not "handbag kitbag or toolbag"?

Question

Why "douche bag"?
Douche is like the italian word "doccia", the original meaning is" tube".

the explanations in the dictionary are 
1 a : a jet or current of liquid (as a cleansing solution) directed against or into a bodily part or cavity (as the vagina) b : an act of cleansing with a douche 
2 : a device for giving douches 
3 British : an abrupt often chastening shock to the nerves, emotions, or awareness

so.. basically it's nothing related to an unattractive or offensive person...
Why "douche bag" ? not "handbag kitbag or toolbag"?

Answer

I found these origins 
http://dialectblog.com/2011/07/01/on-the-evolution-of-douchebag/
and http://askville.amazon.com/origin-insult-douchebag/AnswerViewer.do?requestId=12541998

Answer

You bet James!!! congratulations!

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The word douche comes from French, which means shower. In English, however, the term is specifically describes thoroughly washing the inside of the vagina. It is often considered an unhealthy process.

In pharmacies, douche kits can be bought. I assume they include a bag or something. But anyway, it's basically describing a bag of dirty water that was used to wash a vagina.

Although it sounds quite gross, "douche bag" is not incredibly severe. It's often used to describe a person who is very 骄傲 (which I've somehow momentarily forgotten how to say in English) and is overall a total jerk.

I'm guessing you found the weird on my WeChat, huh? :D

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good to know` thank you

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"Tool" can also be used as an insult roughly equivalent to 'Douchebag'. As in "That guy is a real tool"